Enzyme-catalyzed condensation reactions which initiate rapid peptic cleavage of substrates. 1. How the structure of an activating peptide determines its efficiency.
The addition of a small peptide can significantly increase the rate at which pepsin cleaves a substrate at pH 4.5. Why? In order to find out, we have determined spectrophotometrically the relative ability of over a dozen peptides to speed the initial rate of disappearance of Phe-Trp-NH2 and Leu-Trp-Met-Arg. Here are some of the criteria which establish the reliability of the acquired kinetic data: (1) rates depend linearly on [E] and , to a good approximation, on [activator], (2) measurements with both substrates yield the same ranking for the activators tested; (3) high-pressure liquid chromatographic investigations independently confirm conclusions derived from the spectrophotometric studies. The best activators found were Z-Ala-Phe and Ala-Leu. At 3.2 mM they are respectively 60 and 30 times more effective than an equal concentration of A-(Ala)2. The two-step mechanism given below (for Phe-Trp-NH2) best explains the structural specificity found, as well as other observations on the nature of these activated cleavages. It assumes that reaction commences when pepsin catalyzes synthesis of a peptide bond between activator and substrate. The polypeptide so formed subsequently undergoes scission at a different bond. The modified activator liberated, here designated Z-AA2-AA1-Phe, can eventually provide a variety of reaction products, as the succeeding paper demonstrates.
D000595:Amino Acid Sequence; D004789:Enzyme Activation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008433:Mathematics; D009842:Oligopeptides / Q000494:pharmacology*; D010434:Pepsin A / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
D009842:Oligopeptides; D010434:Pepsin A